1. Field of the Invention
This invention relates to the field of underwater breathing apparatus (UBA), more specifically to self-contained, closed-circuit underwater breathing apparatus, which operates without need of air or breathing gas from an outside supply remote from the diver, and wherein carbon dioxide gas (CO.sub.2) generated by a diver is constantly removed and oxygen (O.sub.2) needed for metabolism is constantly supplied.
2. Description of the Prior Art
A closed-circuit UBA is a form of self-contained underwater breathing apparatus (SCUBA) in which a diver's breathing gas is recycled through a closed loop, adding oxygen and removing carbon dioxide gases as needed. The carbon dioxide gas is typically removed by chemical absorption. UBA typically comprise an oxygen supply bottle, a canister containing CO.sub.2 absorbent, a breathing bag or flexible volume element, connecting hoses, a mouthpiece for the diver, and a diluent gas or inert gas bottle. Inert gases such as helium or helium/nitrogen mixtures are often used. Some UBA versions monitor O.sub.2 electronically and add oxygen when inspired O.sub.2 concentrations drop below desired levels. The only mechanical adjustments that can be made by a diver involve the degree of filling of the breathing bags. As ambient pressure changes, for example as the diver goes deeper or rises, the volume of gas within the breathing bag(s) contracts or expands respectively, and diluent gas may be added or dumped, either manually or automatically. Some UBA have a single breathing bag; others have one for the inhalation side and another for the exhalation side of the recirculating loop.
The closed-circuit breathing apparatus and its basic construction and principles of operation have been known for some time (U.S. Pat. No. 3,837,337 to LaViolette, 1974). Improvements to such equipment are always being made for the diving community, which includes military, commercial underwater construction and salvage, and sport divers. For example, improvements in the control of air or breathing gas flow within the apparatus are discussed in U.S. Pat. No. 4,440166 to Winkler, et al, (1984), wherein particularly with respect to emergency mechanical control system in the event that the electronics fails.
The large majority of patents in the art center around the two critical performance factors for UBA, namely CO.sub.2 absorption and O.sub.2 control. Almost none deal with improving or lessening the difficulty of breathing-particularly during arduous exercise or heavy work. Those that do deal only with the fluid mechanical flow resistance and not other sources of impedance or mechanical resistance to breathing arising from the elements in the UBA including tubing or hoses, canister, etc. In general, breathing resistance in a UBA can be significant for the diver; it can reduce his effectiveness or the duration of work capability, and may, more seriously, contribute to loss of consciousness.
The mechanical resistance to breathing on a UBA is complex, because the breathing is sinusoidal or periodic in nature and not a steady flow. In this kind of flow situation dynamic analyses must be employed, such as those common to the art and discussed in detail by R. Peslin and J. J. Fredberg, "Oscillation Mechanics in the Respiratory System, chap. 11 in Handbook of Physiology: vol III, The Respiratory System, A. P. Fishman (ed.), 1987, American Physiol. Soc., Bethesda, Md., and H. D. Van Liew, "The Electrical-Respiratory Analogy when Gas Density is High", Undersea Biomedical Research, vol. 14, no. 2 (1987) pp. 149-160. In a periodic flow, allowance must be made for additional resistances to the motion of the breathing gas. These resistances are termed elastance and inertance and they cause increased energy loss, because the diver must overcome them as well as the resistance due to flow to keep breathing. Inertial resistance or inertance arises from accelerations and decelerations in the gas flow and displaced water due to the periodic nature of the flow. This generally becomes important only at high frequencies that are generally much higher than human breathing patterns, even under sustained work loads. Inertance increases with depth due to the gas density increasing, but rarely contributes to impedance at the breathing frequencies normally used by a diver.
Elastic resistance or elastance arises from pressure changes as a result of flow entering a closed volume or pressure changes as volume changes (in submersed breathing bags for example). Because of the oscillatory or periodic nature of the flow, complex algebra must be used to describe the overall resistance to flow, which is termed the impedance. Therefore; EQU Z=R-(jE/.omega.)+j.omega.I Eqn. 1
where Z is the impedance in units of pressure/flow rate, R is the resistance due purely to flow (the flow resistance) in the same units, E is the elastance in units of pressure/volume, I is the inertance in units of pressure/flow acceleration, and .omega. is the radian frequency in units of reciprocal time. Eqn. 1 applies to a series arrangement of R, E and I, typical of UBA. Impedance is composed of a real part, the flow resistance, and an imaginary part, which is a combination of the inertance and the elastance. The magnitude of Z can be computed by; EQU .vertline.Z.vertline.=.sqroot.[R.sup.2 +(.omega.I-E/.omega.).sup.2 ]Eqn. 2
so that all three components of impedance contribute to overall energy loss in the flow system. The energy loss is reflected in the pressure drop required to cause flow.
The foregoing are terms of the art necessary to understand the present invention, but they do not constitute the invention.
Flow resistance is present at all times by virtue of the flow rate of gas being moved. The pressure drop required to cause flow at a given rate increases with flow rate in a complicated manner when flow resistance is present. Elastance contributes to the impedance at low frequencies which are generally in the range of a diver's breathing frequencies, whether the diver is at rest or working. It is therefore a major part of the impedance of the UBA. As breathing frequency increases the flow resistive part becomes increasingly more important and elastance increasingly less important factor in the pressure required to maintain flow.
Impedance in the UBA adds to the positive and negative respiratory pressures that a diver must generate to breathe. Impedance is generated by the elastance, inertance and resistance in the UBA. Resistance in UBA arises from breathing hoses, valves, changes in flow diameter, the canister and other similar obstructions in the flow path. Resistance is the fluid mechanical cost of moving the fluid at a given rate. The ratio of the pressure required to drive the flow and the volumetric flow rate is termed the flow resistance.
Elastance is the reciprocal of compliance in the system and is derived in UBA primarily from changes in volume of the breathing bag when immersed. If these changes in volume lead to a vertical expansion or contraction of the bag, pressure is altered by hydrostatic forces, wherein the pressure change (.DELTA.P) is given by; EQU .DELTA.P=.rho.g.DELTA.h Eqn. 3
where .DELTA.h is the vertical displacement and .rho. is the density of the ambient fluid, usually water or sea water, and g is the acceleration of gravity. The shape of the bag and its orientation in the water have an effect of the elastance. Reference is made to D. D. Joye, J. R. Clarke, N. A. Carlson and E. T. Flynn, "Formulation of Elastic Loading Parameters for Studies of Closed-Circuit Underwater Breathing Systems", NMRI Technical Report 89--89, Bethesda, Md., 1989 (report available from NTIS, Washington, D.C.). Elastance is inversely proportional to the cross-sectional area that is perpendicular to the vertical direction. If the volume change in the breathing bag had no vertical component, for example if the bag changed shape in a horizontal direction only, there would be no change in pressure due to volume change and hence no elastance (except for some possible hydrostatic change in an uneven bag fill). There are other contributions to elastance in a UBA, for example the volume of internal hoses and containers in the breathing loop, that have an additional, but much smaller, effect.
The force that moves the flow of breathing gas is respiratory pressure. Although the respiratory pressure imposed by UBA elastance can be relatively high at the low frequencies commonly encountered in a diver's breathing pattern, which is typically in the range 5-60 breaths/minute, particularly 10-40 bpm, there have been no efforts to either statically or dynamically reduce UBA elastic impedance to make it easier for the diver to breathe.